purchases (
  id SERIAL PRIMARY KEY,
  customer_id INT,
  purchase_date DATE,
  amount NUMERIC
)

WITH ordered_purchases AS (
  SELECT
    customer_id,
    purchase_date,
    ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY purchase_date) AS rn
  FROM purchases
),

first_second_purchases AS (
  SELECT
    p1.customer_id,
    p1.purchase_date AS first_purchase_date,
    p2.purchase_date AS second_purchase_date
  FROM ordered_purchases p1
  JOIN ordered_purchases p2
    ON p1.customer_id = p2.customer_id
   AND p1.rn = 1
   AND p2.rn = 2
)

SELECT *
FROM first_second_purchases
WHERE second_purchase_date <= first_purchase_date + INTERVAL '7 days';